2-Methylnaphthalene is a useful compound as an intermediate material for the synthesis of dyes, medicines and the like. Also, this compound has recently been drawing attention as an intermediate material for the synthesis of 2,6-naphthalenedicarboxylic acid which is a monomer of highly functional resins.
Since 2-methylnaphthalene is contained in a methylnaphthalene cut or fraction which is obtained by distillation of tar resulting from carbonization of coal, this compound has been produced in the prior art processes by recovering it from such a methylnaphthalene fraction by means of crystallization or distillation after extracting and removing basic materials. The methylnaphthalene fraction freed from basic materials for use in the crystallization or distillation contains not only 2-methylnaphthalene but also a large quantity of 1-methylnaphthalene which, as a matter of course, remains in a large amount in a recovery solution after crystallization or distillation of 2-methylnaphthalene. Though 1-methylnaphthalene finds some use in such applications as dyes and the like, its demands for industrial use are not high in comparison with the case of 2-methylnaphthalene. In consequence, several processes for the efficient production of 2-methylnaphthalene have been proposed in which 1-methylnaphthalene remained in a recovery solution after crystallization or distillation of 2-methylnaphthalene, or contained in a starting material before the recovery process, is isomerized into 2-methylnaphthalene.
It is known that isomerization of alkylaromatics is promoted by acid catalysts including homogeneous acid catalysts such as AlCl.sub.3, HF--BF.sub.3 and the like and heterogeneous counterparts such as silica alumina, zeolites, modified zeolites and the like. As an example of the application of a homogeneous acid catalyst, Japanese Patent Publication No. Hei 01-13454 discloses a process in which selectivity of 2-methylnaphthalene is improved by using BF.sub.3 --H.sub.3 PO.sub.4. As to the application of heterogeneous acid catalysts, improvement of 2-methylnaphthalene selectivity by the use of partially ion-exchanged Y zeolites has been reported by V. Solinas et al. (Applied Catalysis, vol. 9, pp. 109-117, 1984).
In the prior art processes, isomerization of 1-methylnaphthalene has been carried out making use of these acid catalysts, but with some disadvantages.
For instance, when the aforementioned BF.sub.3 --H.sub.3 PO.sub.4 is used as a homogeneous acid catalyst, its handling is attended with possible danger of inflicting drug injury or toxication. In addition, though H.sub.3 PO.sub.4 in this catalyst can be used again, the expensive BF.sub.3 portion is dissolved in the product and decomposed by water. In consequence, utilization of such a homogeneous acid catalyst has disadvantages in that not only the catalyst itself is dangerous to handle but also its recovery is not easy.
On the other hand, as described in the foregoing, isomerization of 1-methylnaphthalene into 2-methylnaphthalene can be effected making use of heterogeneous acid catalysts such as silica alumina, zeolites, modified zeolites and the like. For example, deterioration with age in the activity of partially ion-exchanged Y zeolites are described in the aforementioned reference by Solinas et al. According to the results of their experiments, however, the isomerization process could not be applied to industrial production of 2-methylnaphthalene because of too quick deterioration of the catalyst. As can be understood from the just described example, isomerization of 1-methylnaphthalene making use of a solid and acidic heterogeneous catalyst has a problem of quick decrease in the catalytic activity because of markedly rapid deterioration of the catalyst due to the deposition of carbonaceous materials.
Because of these problems involved in the prior art processes for the production of 2-methylnaphthalene by isomerization of 1-methylnaphthalene, development of a process in which a heterogeneous catalyst that can be handled and recovered easily and has a long catalyst life is used has been called for in the related industrial field.